The present invention relates to a revolving fixture for supporting tools or instruments, in particular contact detector tools or other reading instruments in general, which fixture is connected to a carrying structure, generally a movable column of a machine tool or a measuring machine, and which allows rotating a portion which carries, clamped thereon, the tool or instrument, relative to a portion fixed to the carrying structure; the said rotation takes place about a main axis of the fixture, which axis generally coincides with the axis of the column of the machine, so as to allow to position as desired the tool or instrument. In fact, for example during the stage of scanning a surface of any shape, by means of a contact detector tool, the tip of the tool should preferably be always perpendicular to the surface to be scanned, and therefore it is necessary to have the possibility to rotate the tool itself to position it in the suitable position.
Therefore, fixtures for the aforementioned purposes have been proposed, which generally are provided with a sliding fit between the surface of the portion fixed to the support and the surface of the rotatable portion carrying the tool. Such fixtures, however, have the disadvantage that they are not able to ensure the obtainment, with a precision in the order of one .mu.m, of the coincidence of the tip of the tool or instrument with the said main axis of rotation, so as to have the said tip always aligned with the said axis of rotation during the detection while the tool is rotating, and thus be always sure that the position of the tip is determined with a high precision, so that no reading errors due to asymmetries will occur; another disadvantage of these fixtures consists in that they are not able to ensure, with the said precision degree in the order of one .mu.m, the position stability of the tip of the tool.
These disadvantages are due to the fact that the said prior art fixtures are not provided with a sufficiently responsive system for adjusting the position of the tip of the tool, and, moreover, are subject to clearance errors, etc., which do not ensure the stability of the tip of the tool.